Plankton diversity and dynamics in a polluted eutrophic lake, Ranchi

The species diversity of a cultural eutrophic lake at Ranchi was studied in relation to external variables (forcing functions) and internal or state variables. The lake receives daily detergent inputs in the form of washings of a variety of objects. A model was constructed for the estimation of detergent inputs from the increase in the phosphate concentration, and from changes in the concentration of inorganic carbon. Nutrients such as inorganic carbon, nitrates, phosphates, sulphates were found to be high in contrast to natural unpolluted systems. The DOM, COD and BOD were also found to be high suggesting organic pollution of the system with an organic carbon load of 5.4 m moles I-1. The growth and development of the plankton constituents was studied in this regime. The natural planktonic rhythm was found to be modified by the polluted condition existing in the lake. The phytoplankton exhibited four peaks in March, May August, and November while, the zooplankton showed three peaks in February, July and October The abundance of zooplankton during the annual cycle oscillated with that of the phytoplankton. There was much more evenness in the zooplankton population in comparison to the phytoplankton. Analysis of both, the zooplankton as well as the phytoplankton population was done using the Bray-Curtis dissimilarity index, importance value index and Shannon-Weaver diversity index. The importance value index was found to provide a better evaluation of the plankton community than the diversity index. The phytoplankton population showed no correlation with nutrient availability as indicated by the correlation-regression analysis and the planktonic rhythm was not in tune with normal unpolluted conditions. The lake was classified as meso-polysaprobic using biological and chemical indices (Pantle and Buck index:3.5, BOD:60; DOM:9.3 and COD: 130).